The present invention relates to cellular telephone systems, and particularly to mobile station transmit power control.
It is well-known that cellular communications systems include base stations arranged geographically in a cellular structure to allow mobile radio stations to communicate with one another and with associated switching systems. Communications occur between mobile stations and base stations, and from the base stations to switching nodes. The switching nodes and control nodes communicate through gateways to other communications systems.
In a Code Division Multiple Access (CDMA) mobile communications system, mobile radios can communicate with a common base station using the same radio frequency band because each mobile station employs a different modulation code that distinguishes its communications from those of other mobile radios.
One consequence of the mobile radios in a common cell transmitting on a common frequency is the possibility of interference caused by one mobile station""s transmission to another mobile station""s transmission. Factors (such as environmental factors proximity, etc.) affect signal strength disparities between multiple mobile radios communicating with a common base station, resulting in interference between xe2x80x9ccompetingxe2x80x9d radios. As a result, some mobile radio communications with a communication base station have the potential to dominate others on the same radio frequency band.
In an ideal system, each mobile radio sends signals to a common base station at a power level that ensures all of the different mobile-transmitted signals arrive at the base station with about the same average power that yield an appropriate signal to interference level for each mobile station using the same frequency. But, because the environment of the mobile station, the proximity of the mobile stations, etc. are changing, power levels for all radios is vigilantly controlled. Controlling those power levels, called uplink (or reverse) transmit power control (TPC), from the mobile station to the base station is a significant factor in improving the performance and capacity of a CDMA system.
Several methods are known for controlling uplink power including control based on estimated path loss (or other suitable measure) of signals received from a base station (open loop transmit power control). Another method is for the base station to send transmit power control messages to the mobile station (closed loop power control) based on comparative signal strength received at the base station.
U.S. Pat. No. 5,774,785 (commonly owned) describes one such uplink control method in which a mobile radio restricts its transmission power to a range defined by a maximum and minimum transmission power. U.S. Pat. No. 5,390,338 (commonly owned) describes another transmission control method that relies on the distance between the transmitter and receiver. Other prior art address, in various contexts, how power levels are set to minimum levels to achieve desired performance goals and maximum levels to comply with communications regulations.
Mobile stations are constrained by their design and other factors to practical maximum and minimum power outputs based on their dynamic ranges. A mobile station cannot transmit at power levels outside of its range. This can be a particular problem in situations where a mobile station is very close to a base station and its uplink power level is quite high at the receiving base station. As described previously, ideally a base station receives all transmissions from all assigned mobile radios at about the same power level that yield an appropriate signal to Interference level to avoid interference caused by a dominating mobile radio transmission. When a mobile radio begins to dominate the uplink transmissions of neighboring mobile radios, the mobile receives an instruction from the base station (or generates an internal instruction) to lower uplink transmission power to a more balanced level. A problem occurs, however, when a mobile radio is so close to its serving base station (and thus its received uplink power level is so high) that even at the mobile""s minimum output transmit power, the large power level received from the dominating mobile station at the base station is still so large that it causes unacceptable levels of interference. That ultimately may result in a decrease in capacity and coverage at the base station.
One possible solution to the above problem is to increase the dynamic range of the dominating mobile stations. But, that is a physical design change that does not provide immediate relief.
The present invention solves the problem by determining the maximum power level that may be permissibly received by the base station to ensure maximum capacity and acceptable levels of interference. The maximum power reception level may be broadcast by the base station over a control channel or the mobile station may determine the maximum allowed level of reception based on signal-to-interference target values defined for that mobile station. If the mobile station determines that its lowest transmit power level will be received at the base station at a level that exceeds the base station""s maximum receive power level, the mobile station deactivates its transmitter. Alternatively, the base station could also detect an excessive receive level and order that mobile station to deactivate its transmitter. Ultimately, the transmission from the mobile station may resume if the received power level from that mobile decreases to acceptable levels, e.g., the mobile moves away from the base station. Otherwise, it may be necessary to release the call.
While much of the relevant prior art tries to optimize transmit power levels in some range in order to minimize interference while ensuring sufficient quality of communication, none propose that a mobile station transmitting at its lowest transmit power level to a base station that is receiving it above the maximum receive level of the base station, stop transmitting to eliminate the interference caused by the mobile""s high received power level (even though the mobile is transmitting at its lowest transmit power level). Nor is there any known teaching of specific techniques in this context where a base station or a mobile station determines when the mobile is to stop transmitting or to eventually release the call.